Since the construction of the first laser by Maiman in 1960 [1] one important scientific and technological goal in the field was to increase the power delivered by the laser beam and to explore novel phenomena that only occur for such high electromagnetic field intensities. The solution came with the pulsed lasers operating in the mode-locked regime, where the energy of the pulse is emitted in a very short temporal event. Nowadays, lasers with femtosecond (1 fs=10−15 s) pulse durations can generate peak powers of the order of a Petawatt (1 PW=1015 W). Optical pulses with durations ranging from a few optical cycles to hundreds of fs are so short that no direct method for their measurement exists. To this purpose, techniques based on nonlinear optical interactions (autocorrelation or cross-correlation diagnostics) are usually implemented. Although these methods can provide a good measurement of the pulse duration, they do not generally provide complete information about the spectral phase of the pulse that ultimately determines the pulse shape and duration. The complete characterization of such short events is therefore very important and often challenging.
Several methods that combine autocorrelation and spectral measurements have been proposed to overcome this issue and to obtain amplitude and phase reconstruction of the pulses [2-4]. Nowadays, the most used methods are different versions of either Frequency Resolved Optical Gating (FROG) or Spectral Phase Interferometry for Direct Electric Field Reconstruction (SPIDER). The FROG method relies on spectrally resolving time-gated signals and creates a spectrogram-like trace by spectrally resolving an autocorrelation signal and enables complete characterization of a given pulse by means of an iterative algorithm applied to the trace [5, 6]. On the other hand, the SPIDER method relies on interferometry in the spectral domain: the spectrum of a given pulse is made to interfere with a time and frequency shifted replica of itself, and the resulting spectral interferogram is recorded [7-9]. Both methods can provide very good results for pulses in the range of 20-200 fs. However, standard FROG and SPIDER devices are normally very sensitive to alignment and to phase-matching bandwidth requirements. Even if recent SPIDER-related methods have partially overcome this issue, in all of the above techniques the characterization of few-cycle laser pulses is still challenging and usually requires specific tuning and materials in order to accommodate the associated broad bandwidths of the pulses.
Another method for pulse characterization based on phase scanning, known as Multiphoton Intrapulse Interference Phase Scan (MIIPS) [10], was more recently introduced. A set of known spectral phases is applied to the pulse to be characterized, most usually via an active pulse shaping device, and the resulting second harmonic generated (SHG) signals are measured. By finding which locally introduced amount of group delay dispersion (GDD) results in compression at a given wavelength, an approximation to the original GDD of the pulse is directly obtained from a contour plot without the need of any mathematical retrieval procedure [11-13]. The pulse-shaping device is then programmed to introduce a GDD opposite to the measured one, and the whole experimental and numerical process must be repeated until a given spectral phase is achieved.
A more recent method is Self-Referenced Spectral Interferometry (SRSI), where a reference pulse with a flat spectral phase is collinearly generated from the input pulse by cross-polarized wave generation (XPW) in a nonlinear crystal. The spectral interference pattern resulting from the combination of the input pulse and the reference pulse allows direct retrieval of the spectral phase and intensity. This method however can only measure pulses with durations very close to the Fourier limit, and no more than 2 times this limit. Therefore, SRSI has a very limited tolerance to the input pulse chirp and a small measuring range compared to most other techniques. On the other hand, it can only measure amplified laser pulses, since XPW is a third-order nonlinear process that requires several micro Joules of energy per pulse in order to work.
A recently proposed method called dispersion-scan (d-scan) can retrieve the phase of ultrashort laser pulses by applying a set of known spectral phases by progressively inserting a wedge within a chirped mirror and wedge pair compressor and measuring the corresponding spectra of a nonlinear signal, such as the second harmonic generated in a phase-matched nonlinear crystal. Pulse retrieval is performed via a holistic iterative algorithm [14-16]. In the d-scan method a pulse compressor is used as part of the diagnostic tool itself. This method is very simple and robust compared with FROG or SPIDER. However, the implementation based on chirped mirrors and wedge compressor requires the phase to be scanned over the set of applied dispersion values by progressively moving one of the wedges. This approach works very well provided that the pulse train emitted by the laser has a stable spectrum and spectral phase, but cannot work in single-shot configuration, where measurement of all the data needed for the pulse reconstruction must be recorded in a single measurement and for a single pulse. The d-scan method requires several successive experimental steps, corresponding to different wedge insertions, to record all the data needed for the phase reconstruction. Single-shot methods are crucial for the characterization of the pulses provided by high power lasers with low repetition rates.
Therefore, the introduction of a new system (and method) that is compact, robust; less sensitive to alignment and wavelength, less expensive compared to existing technologies, while being capable of characterizing ultrashort laser pulses by recording all the data needed for pulse reconstruction in a single-shot configuration, is in high demand for ultrashort laser pulse development and applications.